Many kinds of flat glasses are being used in various fields like window panes, window screens of vehicles and mirrors. Such a flat glass may be manufactured in various ways. Among them, a representative method is a production method using a float method. For example, thin glass planes or glass films for TFT displays are frequently manufactured by the float method. The glass manufactured by the float method is called a float glass.
FIG. 1 is a schematic diagram showing a float bath for manufacturing a float glass.
As shown in FIG. 1, a float glass is generally formed by using a float bath 10 where a molten metal M such as molten tin or molten tin alloy is stored and flows. At this time, the float bath may include a bottom portion 12 located at a lower portion and a loop portion 11 located at an upper portion, and a side seal (not shown) may be interposed between the bottom portion 12 and the loop portion 11.
In the float bath 10, a molten glass G having a lower viscosity than the molten metal M and lighter than the molten metal M by about ⅔ is successively supplied into the float bath 10 through an inlet thereof. In addition, the molten glass moves to the downstream of the float bath 10 while floating and spreading on the molten metal M. In this process, the molten glass nearly reaches an equivalent thickness according to its surface tension and gravity to form a glass strip or ribbon which is solidified to some extent. In the float bath 10, the thickness of a produced glass ribbon may be changed by adjusting or changing an amount of glass put through the inlet, a pulling speed determined by a rotation speed of rollers, a molding component such as top rollers installed in a float chamber, or the like.
Meanwhile, the molten glass ribbon formed in the float bath 10 as above is transferred to an annealing furnace by a roller 30 adjacent to the outlet of the float bath 10 and experiences an annealing process.
The float glass manufacturing method includes cyclic successive processes and may operate constantly without a cessation, which allows flat glasses to be manufactured for several years without a pause. For this reason, the float glass manufacturing method is in the limelight as a representative flat glass manufacturing method.
When manufacturing a float glass by using the float bath 10, temperature control in the float bath 10 is very important. In particular, since a molten metal M is contained in the float bath 10 and a molten glass G floats on the molten metal M, the inside of the float bath may be maintained in a high temperature state, for example 1300° C. to 600° C. In addition, since the molten glass on the molten metal spreads into a flat plate while progressing to the downstream of the float bath and should be solidified to some extent, the float bath 10 is configured so that its temperature gradually lowers from the upstream to the downstream.
As described above, the inside of the float bath 10 has a high temperature and may also have local temperature differences. In addition, since the float glass is successively formed, the internal temperature of the float bath 10 needs to be constantly controlled.
In general, the internal temperature of the float bath 10 may be controlled by a heater 20 and a cooler. Here, a plurality of heaters 20 may be provided at a loop portion 11 of the float bath 10 to supply heat into the float bath 10. In particular, the heaters 20 may supply different heat quantities individually or by group, so that a temperature difference is formed according to a location of the float bath 10. For example, the inside of the float bath 10 may be classified into a plurality of areas, namely control zones, and at least one heater 20 may be installed in each control zone to supply heat to the corresponding area.
However, in the case individual heaters 20 or individual heater groups give different heat quantities to form a temperature difference according to locations of the float bath 10, it is not easy to control temperature according to locations of the float bath 10. In particular, one of the most important factors which make it difficult to control temperature according to locations of the float bath 10 is heat transfer such as heat radiation transferred from another adjacent heater 20. For example, even though the inside of the float bath 10 is divided into several control zones and different heat quantities are supplied by heaters 20 installed in the control zones, a specific control zone may be supplied with heat not only from a heater 20 installed in the corresponding control zone but also from a heater 20 installed in another adjacent control zone.
Therefore, in the existing float bath 10, it is very difficult to control temperature according to locations of the float bath 10. In addition, if the temperature of the float bath 10 is not suitably controlled, a bad influence may be given to the quality of a glass, which may lower a glass production yield and increase manufacture time and costs.